In recent years, electrophotography has played a major role in the field of copying machines, printers, facsimiles, etc. because it advantageously provides a high printing speed and a high print quality.
Electrophotographic photoreceptors comprising an inorganic photoconductive material such as selenium, selenium-tellurium alloy and selenium-arsenic alloy are widely known as the electrophotographic photoreceptor for use in electrophotography. On the other hand, extensive studies have been made on electrophotographic photoreceptors comprising an organic photoconductive material which is advantageous in cost, productivity and disposability as compared to these inorganic photoreceptors. Up to the present, these organic electrophotographic photoreceptors have surpassed the inorganic photoreceptors. In particular, an electrophotographic photoreceptor having a function-separation type laminated structure in which the photo-induced generation of electric charge and the transportation of electric charge, which are elementary processes in the photoconduction, are carried out by separate layers have been developed. This type of an electrophotographic photoreceptor provides an increased degree of freedom in selecting materials to thereby exhibit a remarkable enhancement of properties. At present, this function-separation type laminated organic photoreceptor becomes the main current. A film obtained by vacuum-evaporating a pigment having charge-generating ability such as quinone pigments, perylene pigments, azo pigments, phthalocyanine pigments and selenium, or by dispersing the above described pigment into a binder resin in a high concentration, is practically used as the charge-generating layer in the function-separation type laminated organic photoreceptor. On the other hand, a layer comprising an insulating resin and a low molecular weight compound having charge-transporting ability such as hydrazone compounds, benzidine compounds, amine compounds and stilbene compounds molecularly dispersed therein is used as the charge-transporting layer.
A photoreceptor to be mounted on an analog electrophotographic copying machine which operates by optically forming an image of the original onto the photoreceptor followed by exposure of the image is required to have photo-induced potential decay characteristics shown in FIG. 1, that is, to undergo potential decay in proportion to exposure amount (Hereinafter, a photoreceptor of this type is referred to as a "J-character type photoreceptor") so as to provide a good reproduction of half tone in the density gradation. All of the above described inorganic photoreceptors and function-separation type laminated organic photoreceptors exhibit photo-induced potential decay characteristics falling within this category. On the other hand, digital electrophotographic apparatuses which have been extensively studied with the recent requirement for higher image quality, higher value added and development of network generally employ an area gradation system which provides gradation by percent area such as dot. Therefore, the digital electrophotographic apparatus preferably employs a photoreceptor having so-called S-character type photo-induced potential decay characteristics (hereinafter referred to as "S-character type photoreceptor") in which potential shows no decay until a predetermined exposure amount is reached but shows a rapid decay when exposure amount exceeds the predetermined value for enhancing pixel sharpness.
The S-character type photo-induced potential decay characteristics are known phenomena with a single-layer photoreceptor comprising an inorganic pigment such as ZnO or an organic pigment such as phthalocyanine dispersed in a resin as disclosed in R. M. Schaffert, "Electrophotography", Focal Press, page 344, 1975; and J. W. Weigl, J. Mammino, G. L. Whittaker, R. W. Radler, J. F. Byrne, "Current Problems in Electrophotography", Walter de Gruyter, page 287, 1972. In particular, many single-layer photoreceptors for laser exposure have been proposed which comprises a resin and a phthalocyanine pigment dispersed therein and is sensitive to near infrared range, which is the emission wavelength of semiconductor lasers which are often used (as disclosed, for example, in Guen Chan K., Aizawa, "Journal of Japan Society of Chemistry", page 393, 1986; JP-A-1-169454 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-2-207258, JP-A-3-31847, and JP-A-5-313387). However, such a single-layer photoreceptor needs to necessitate a single material fulfill both the two functions, i.e., generation of electric charge and transportation of electric charge. Nevertheless, materials which can fulfill both the two functions are rare, and such materials which can be practically used have never been obtained. In particular, a particulate pigment generally has many trap levels and thus is disadvantageous in that it has a low charge-transporting ability or tends to keep residual charge therein. Thus, such a particulate pigment is not suitable for transportation of electric charge. Only one exceptional practical example is a single-layer photoreceptor comprising a resin and ZnO dispersed therein. This type of a single-layer photoreceptor makes the best use of the hydrophilicity of ZnO to find a good application to a master plate for offset printing process which comprises plate-making by an area gradation process in which an image is formed in accordance with the presence or absence of adhesion of a hydrophobic toner (as disclosed in Kawamura, "Base and Application of Electrophotography", Society of Electrophotography, Corona, page 424, 1988). However, the success of this type of a single-layer photoreceptor was achieved only because it is applied to a master plate in which requirements for high printing speed and press life are not so high. Thus, this type of a single-layer photoreceptor cannot be practically used as a photoreceptor for copying machines and printers, which is the technical field of the present invention. In view of the above, it is desired to introduce the function-separation type laminated structure into the S-character type photoreceptor to enhance the degree of freedom in selecting materials and hence to enhance the comprehensive properties of the photoreceptor.
In respect to this problem, D. M. Pai et al. reported that a laminated photoreceptor consisting of a charge-generating layer and a charge-transporting layer wherein the charge-transporting layer is a nonuniform charge-transporting layer comprising at least two charge-transporting regions and at least one electrically inactive region, the charge-transporting regions coming into mutual contact to form a contorted charge-transporting passage, can realize S-character photo-induced potential decay characteristics when combined with an arbitrary charge-generating layer (as disclosed in JP-A-6-83077 (U.S. Pat. No. 5,306,586)). However, even this type of a photoreceptor must necessitate the charge-transporting layer fulfill the function of exhibiting S-character type photo-induced potential decay characteristics and the function of transporting electric charge. As compared to the charge-transporting layer in the conventional laminated photo-induced photoreceptor having J-character type photo-induced potential decay characteristics, the charge-transporting layer in this type of a laminated photoreceptor must be imparted the additional function of exhibiting S-character photo-induced potential decay characteristics. Thus, this type of a laminated photoreceptor is still subjected to a restriction in degree of freedom with respect to the design of the charge-transporting layer. The present invention is to solve the above described problems.